A device for compensating for pressure drop in a product pipe through which a liquid flows is used, for instance, in the food industry for filling containers in a filling machine.
The filling machine is connected to the product pipe and has a filling station, at which liquid contents, such as milk or fruit juice, are introduced into the empty containers. To this end, the filling station usually comprises a filling pipe which via a filling valve is connected to the product pipe. For filling a container the filling pipe is inserted into the container, after which the filling valve is opened. When the filling valve is opened, liquid will thus flow through the filling valve and into the container. When a required volume has been transferred to the container, the filling valve is closed. The filling valve can be, for instance, time- or volume-controlled. Then the filled container can be conveyed to a sealing station, and a new empty container can be conveyed to the filling station.
When a filling valve is opened, a pressure drop arises in the product pipe. This pressure drop may cause more or less serious disturbances.
The pressure drop can propagate to other filling stations which are connected to the product pipe and thus affect the filling processes which are controlled by the filling valves of the respective filling stations.
Furthermore the pressure drop can affect the filling process controlled by the filling valve in question. More specifically, the pressure drop affects the rate at which the product flows through the product pipe.
If the filling valve is time-controlled, the pressure drop thus constitutes an uncertainty with regard to the accuracy with which the container can be filled. The container can either be filled with too small or too large an amount of liquid.
If the filling valve is volume-controlled, the pressure drop implies that the filling time required for filling the container increases since the pressure drop causes a reduction of the flow rate of the product in the product pipe.
When the valve is closed after filling, a surge may be generated, propagating through the product pipe. This surge can be detrimental to the filling station and also affect the filling processes at other filling stations connected to the product pipe.
To eliminate, or in any case reduce, these disturbances caused by said pressure drop, a device for compensating for the pressure changes is usually connected to the product pipe.
Such a known device for compensating for pressure drop and pressure changes is the so-called balancing or level vessel, which is a container in which the liquid level is regulated by means of a float which controls the supply of liquid. The liquid level is regulated so as to always be kept within certain limits, independently of the discharge of liquid from the container. When liquid is discharged from the container, this occurs by gravity, and no pressure drop or surges that can propagate will arise, either at the beginning or at the end of the discharge. The construction, however, suffers from the drawback that it is difficult to clean and sterilise, which are important aspects in systems for filling of containers with liquid food contents. The construction is also relatively expensive to manufacture and install.
SE 410,844 discloses a device for compensating for surges in a product pipe through which a liquid flows, which device partly solves the problems of the above device as regards cleaning and sterilising. The device comprises a first and a second chamber, adjoining each other and being separated by means of a flexible wall. The first chamber has an inlet and an outlet to be connected in series to the product pipe. The second chamber is connected by a control means to a compressed air source. In case of surges in the product pipe, the wall of the first chamber is stretched in a manner that corresponds to the surge. The pressure in the second chamber is regulated so as to compensate for the change in volume of the first chamber. The pressure drop is taken care of by the first chamber being allowed to collapse.
The device according to SE 410,844, however, has a serious drawback since it has a most limited service life. The reason for this is the repeated stretching of the wall of the first chamber which is caused by said surges. Up to now, it has proved difficult to find a material which with an acceptable service life can be subjected to such strain.
U.S. Pat. No. 6,264,069 discloses a device for dampening surges from liquids. The device consists of an inner and an outer tube, the inner tube being connected to, for instance, a filling system for beverages. The device is intended to dampen surges from a pump in the system. A surge causes the inner tube to expand until it contacts the outer tube, which prevents further expansion.
The device described in U.S. Pat. No. 6,264,069, however, involves problems similar to those of the device of SE 410,844, i.e. the repeated stretching of the inner tube causes it to obtain a short service life.
U.S. Pat. No. 2,838,073 discloses a device which is adapted to dampen surges in a flame thrower. The device of US Pat. No. 2,838,073 consists of a resilient inner tube of heavy rubber and an outer reinforced tube. When a surge arises, it will expand the inner tube to absorb the forces. The outer tube ensures that this expansion is only allowed to occur to a predetermined extent. Also this device has the problem of a short service life since the inner tube is stretched repeatedly.
U.S. Pat. No. 2,875,786 discloses a device for dampening surges from liquids. The device consists of an inner non-flexible perforated pipe, a flexible tube which is arranged immediately outside the pipe and a second, non-flexible tube which encloses the flexible tube. When a surge arises in a system, the flexible tube expands and, thus, dampens the surge. The other tube ensures that said expansion is only allowed to take place to a predetermined extent. Also U.S. Pat. No. 2,875,786 has problems similar to those of U.S. Pat. No. 6,264,069 and U.S. Pat. No. 2,838,073 since the flexible tube is stretched repeatedly.
JP 3,175,599 discloses a hose for pressurised systems. The hose comprises an inner layer and an outer layer. A reinforced layer is arranged on the opposing sides of the respective layers. A plastic-reinforced layer is arranged between the reinforced layers. The plastic-reinforced layer serves only to allow the hose to expand to a predetermined extent. The hose of JP 3,175,599 is not intended to affect the pressure of a system, but still obtains a short service life caused by the repeated stretchings.
There is thus a need for a less expensive and easily cleaned device which can compensate for pressure drop in a product pipe and which also has a satisfactory service life.